LED lighting systems generally employ regulated power sources for supplying power to the LEDs. In the art of LED drivers, it is known to use a pulse-width modulated (PWM) drive current as a power source to the LED. Generally, a regulator circuit includes several sub-circuits with active and passive elements that operate in concert to provide power regulation.
A simple circuit diagram for a typical regulator for driving LED strings is shown in FIG. 1. A Buck-Boost converter is formed of Q1, L, D1 and C1. A serial LED string is denoted as D5. The OP-AMP1 along with the surrounding resistors, R5, R6, R7, R8 forms a differential amplifier for the sensed current signal form R1. An analog PID controller is formed by OP-AMP2 along with the surrounding components R9, R10, R11, R12, C5, C6, and C7. A PWM signal is introduced to the regulator circuit through the modulator COMP1. In steady-state DC operation, the LED string D5 current is regulated by the regulator circuit.
FIG. 2 illustrates the regulator circuit configured to provide the LED string D5 with light output adjustment or dimming functionality. It is known to be beneficial to use a low-frequency PWM current for the LED string D5 by invoking a series switch Q2 as is depicted in FIG. 2. In order to reduce the current peak pulse in the LED string D5 at each turn on event, a simple sample-and-hold 210 sub-circuit consisting of R2, R4, C2 and D2 is provided. As shown in FIG. 2, the sample-and-hold sub-circuit has an output voltage V3 and an input voltage V6. It can be shown that when the diode D2 conducts, the transfer function of the sample-and-hold 210 sub-circuit is:
                                                        V              ⁢                                                          ⁢              3                                      V              ⁢                                                          ⁢              6                                =                                    K              ⁡                              (                s                )                                      =                                          K                0                            ⁢                              1                                  1                  +                                      s                    ω                                                                                      ,                            (        1        )                                                      where            ⁢                                                  ⁢                          K              0                                =                                    R              ⁢                                                          ⁢              2                                                      R                ⁢                                                                  ⁢                2                            +                              R                ⁢                                                                  ⁢                4                                                    ,                                  ⁢        and                            (        2        )                                ω        =                                                            R                ⁢                                                                  ⁢                2                            +                              R                ⁢                                                                  ⁢                4                                                                    R                ⁢                                                                  ⁢                2                            ⋆                              R                ⁢                                                                  ⁢                3                            ⋆                              C                ⁢                                                                  ⁢                2                                              .                                    (        3        )            
Inspection of equation (1) reveals that the sample-and-hold introduces a pole, with an associated 90 degree phase delay, into the current regulation loop. The LED regulator phase margin is therefore reduced and the regulator circuit tends to oscillate. It would therefore be desirable to provide an improved LED regulator configuration that addressed these and other limitations.